Positron Lifetime Measurements Research Articles

Thermal evolution of La 3+/ZrO 2 solid solutions obtained by mechanochemical activation

A study on the microstructural evolution with the temperature of La 3+/ZrO 2 cubic solid solutions obtained by mechanochemical activation of mixtures ZrO 2–La 2O 3 is presented. The development of crystallinity with thermal treatments in the range 1073–1673 K was followed by means of positron lifetime measurements and X-ray diffraction as main experimental techniques.

Positrons as chemically sensitive probes in interfaces of multicomponent complex materials: Nanocrystalline Fe90Zr7B3

Abstract The present paper reports on a combined analytical and structural study of nanocrystalline Fe90Zr7B3 by means of positron annihilation, (analytical) high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction. Particular focus is laid on the chemical nature of the intergranular amorphous matrix which occurs between the α-Fe nanocrystallites. Energy-dispersive X-ray measurements (EDX) with an electron nanobeam reveal an increased Zr content at the interface between the nanocrystallites and the intergranular amorphous phase. According to positron lifetime measurements, the intergranular amorphous phase and the interfaces between this phase and the nanocrystallites exhibit structural free volumes of the mean size slightly smaller than a lattice vacancy as in the amorphous precursor material. Coincident Doppler broadening measurements of the positron-electron annihilation photons show that the fraction of Zr in the neighborhood of the structural free volumes is higher in nanocryst...

Postgrowth annealing of defects in ZnO studied by positron annihilation, x-ray diffraction, Rutherford backscattering, cathodoluminescence, and Hall measurements

Defects in hydrothermal grown ZnO single crystals are studied as a function of annealing temperature using positron annihilation, x-ray diffraction, Rutherford backscattering, Hall, and cathodoluminescence measurements. Positron lifetime measurements reveal the existence of Zn vacancy related defects in the as-grown state. The positron lifetime decreases upon annealing above 600 °C, which implies the disappearance of Zn vacancy related defects, and then remains constant up to 900 °C. The Rutherford backscattering and x-ray rocking curve measurements show the improvement of crystal quality due to annealing above 600 °C. Although the crystal quality monitored by x-ray diffraction measurements is further improved after annealing at above 1000 °C, the positron lifetime starts to increase. This is due to either the formation of Zn vacancy related defects, or the change of the Zn vacancy charge state occupancy as a result of the Fermi level movement. The electron concentration increases continuously with increasing annealing temperature up to 1200 °C, indicating the formation of excess donors, such as oxygen vacancies or zinc interstitials. The cathodoluminescence measurements reveal that the ultraviolet emission is greatly enhanced in the same temperature range. The experimental results show that the ZnO crystal quality, electrical and optical characteristics are improved by postgrowth annealing from 600 to 1200 °C. The disappearance of Zn vacancy related defects contributes to the initial stage of improved crystal quality.

Mesoporous low- k hydrogen-silsesquioxane films characterized by positron annihilation and other techniques

Pore characteristics of hydrogen-silsesquioxane low- k films with various porosities were studied by positron annihilation γ-ray spectroscopy and lifetime measurements. From positron annihilation γ-ray spectra, 3 γ annihilation probabilities ( I 3 γ ) of incident positrons were determined. I 3 γ due to the self-annihilation of ortho-positronium escaped out from the film increased with increasing film porosity, indicating the presence of open pores in high porosity films. Positron lifetime measurement showed that the pore is enlarged with increasing film porosity.

Defect characterization of ZnBeSe solid solutions by means of positron annihilation and photoluminescence techniques

Defect characterization of as-grown Zn1−xBexSe compound semiconductors was studied by positron lifetime and photoluminescence measurements. We obtain both experimental and theoretical evidence that the bulk lifetime of free positrons decreases linearly with Be alloying. The average positron lifetime increases with the temperature, indicating that both vacancies and negative ions trap positrons. The decomposition of the lifetime spectra shows that the positron lifetime of the vacancy decreases with an increase in Be content, as predicted by theoretical calculations. The concentration of vacancies and negative ions obtained from positron measurements follows the same trend with Be alloying. A comparison of positron measurements with theoretical calculations and photoluminescence experiments in Zn vapor annealed samples indicates that the vacancy corresponds to a Be vacancy. Besides, the negative ions trapping positrons at low temperatures have been attributed to Be interstitial atoms. The intensity of the B photoluminescence band correlates with the concentration of Be vacancies and Be interstitial atoms. We thus conclude that the electron levels of those defects are involved in the optical transition that leads to the B photoluminescence band.

Role of defects in metal mediated crystallization inAl/a−Gemultilayers

Metal-catalyzed crystallization of amorphous germanium (a-Ge) is investigated to provide experimental evidence that this phenomenon is a defect mediated process. Positron lifetime measurements are carried out on as-grown and annealed Al/a-Ge multilayers, to unambiguously identify the type of defects. Substantial positron trapping occurs at interfacial voids in the as-grown specimens. Annealing at temperatures of 450-533 K results in rapid interdiffusion of the layers, leading to the formation of crystalline precipitates of Ge in the Al matrix. We provide direct evidence that the rapid transport and growth of crystalline Ge precipitates is aided by the transport of defects. These defects are identified to be two/three vacancy clusters. The broader implications of these results to microstructure and phase evolution in thin film multilayers is indicated.

Positron lifetime measurements in sintered alumina

Sintered alumina samples of grain diameter 1.7 and 4.5 μm, in which silicon impurity dominates, i.e. 90 ppm, were investigated by positron annihilation lifetime spectroscopy at room temperature using a new procedure of computer analysis. A bulk lifetime of 122 ± 4 ps was derived. A trapped positron lifetime of 137 ± 2 ps and a trapping coefficient μ ≥ 6 × 1014 s–1 were assigned to defects located within the grain. These traps are likely cationic vacancies VAl″′ and clusters (nSi•Al : VAl″′) induced by SiO2 dissolution into Al2O3. An annihilation process of small intensity and long lifetime, which can reflect annihilation in vacancy clusters located at grain boundaries, was found. The derived bulk lifetime is confirmed by measurements in Al2O3 single crystal of high purity. The reliability of the computer analyses is discussed.

The temperature dependence of free volume in polymethylpentene by positron annihilation

Positron lifetime measurements in polymethylpentene were performed in the temperature range 8.3–299 K. A conventional fast–slow coincidence lifetime spectrometer with plastic scintillators were used. All the measurements were made in vacuum. Additional measurements (X-ray diffraction, differential scanning calorimetry and mechanical spectroscopy) were also performed to establish properties of samples. Mean free volume radii and fractional free volume in the investigated samples were estimated from the positron lifetimes.

Positron lifetime spectroscopic studies of nanocrystalline ZnFe2O4

By carrying out positron lifetime measurements in zinc ferrite (ZnFe2O4) samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the Zn2+ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. ZnFe2O4 samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesized through the hydrothermal precipitation method showed remarkably larger lifetimes for trapped positrons upon nanocrystallization in comparison to the samples prepared through the citrate route.

Positron lifetime study of native vacancy-like defects in chalcogenide glasses

Modified model for positron annihilation in vitreous chalcogenide semiconductors is developed to explain a number of previously obtained results on positron lifetime measurements in glassy As–Ge–S of stoichiometric As 2S 3–GeS 2 and non-stoichiometric As 2S 3–Ge 2S 3 cut-sections.

Theoretical calculation of positron lifetimes for LaNi5–H system

Positron lifetime spectroscopy is a powerful tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process of LaNi5 and excess vacancies becomes mobile and form microvoids by a thermal activation process. For further investigation, theoretical approaches using electronic structure calculations are indispensable. In this work, we have performed theoretical calculations of positron lifetime for LaNi5–H system using first principles electronic structure calculations. By comparison between the theoretical and experimental positron lifetimes, one of the defect components during hydrogen absorption can be ascribed to the annihilation at vacancy-clusters composed of two or three Ni vacancies. The positron lifetime of the vacancy-cluster component increases over 400 ps during isochronal annealing after hydrogen desorption. The vacancy-cluster may contain not only Ni vacancies but also La vacancies, since the vacancy-cluster composed of only Ni vacancies cannot yield such a long positron lifetime.

Median implantation depth and implantation profile of 3–18 keV positrons in amorphous polymers

Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z1/2(E), of 3–18 keV positrons and their implantation profile P(z,E) in a total of 13 thin films of atactic polystyrene, poly(styrene-co-acrylonitrile), and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z1/2(E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I3 of the long lifetime. z1/2(E) was parametrized with the commonly used power-law fit z1/2(E) = (A/)En, with and E in units of g cm–3 and keV, respectively, yielding A = 2.81(±0.2) µg cm–2 and n = 1.71(±0.05). Excellent agreement between amorphous polymer and literature data on Al and Cu suggests that the median implantation depth of positrons for low- to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker et al., the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation. ©2003 The American Physical Society

Mössbauer and positron annihilation studies in plastically deformed Fe 72− xAl 28Ti x ( x=0, 2, 9) alloys

Positron lifetime measurements in deformed Fe–Al–Ti alloys, obtained by filing the homogenized ingots show that monovacancies are created during the filing process. This is in contrast with known results on deformation due to rolling where dislocations have been reported as the major defect type. These results together with those from Mössbauer spectroscopy suggest that the vacancies are dominantly created due to aluminum atoms being displaced to the nearby sites where they replace iron atoms. Annealing at 900°C for extended periods caused defect concentration to greatly reduce. Positron lifetime spectroscopy has been successfully used to detect agglomeration of vacancies to form di- or tri-vacancies for the first time. Addition of titanium is found to facilitate fast removal of defects during controlled heat treatments.

High sensitivity of positron annihilation to thermal oxidation of polyethylene

We demonstrate the high sensitivity of positron annihilation to compositional changes related to the thermal degradation of polyethylene (PE). Positron annihilation γ-ray and lifetime measurements were conducted for PE films with and without antioxidant (1000-ppm Ciba® IRGANOX® 1076), subjected to heat treatment at 100 °C for different periods, to a maximum of 30 days. For the film without antioxidant, the positron Doppler parameter (S) and ortho-positronium formation probability (Io−Ps) appreciably decreased with increased heat treatment times, whereas they barely changed for the film with antioxidant. This, together with the Fourier transform infrared measurements, demonstrated that the variations of S and Io−Ps are caused by the thermal oxidation of PE. The S parameter was found to be sensitive to the early stage of degradation, where the carbonyl concentration is inferred to be lower than 100 ppm. The high sensitivity results from the large positron mobility in PE and from the high positron affinity of oxygen-containing polar groups. This work provides the basis for an application of positron annihilation to sensitive detection of the initial degradation of PE and other nonpolar polymers.

The occupying activities of helium in ball-milled hydrogen-storage alloys

Through the measurements of positron lifetime and analysis of structure, the properties of helium in the ball-milled LaNi4.75Al0.25 and Zr50Co50 alloys were investigated. It was found that the amount of the injected helium increases with increasing helium pressure. It is comparatively easier for helium to enter the LaNi4.75Al0.25 alloy. The difference of the positron lifetime parameters between the above two specimens shows that the properties of the injected helium are different. For Zr50Co50, helium occupies the intersection of two or three crystallite interfaces and/or cluster of vacancies at grain boundaries or groups of vacancies firstly. With increasing pressure of the helium atmosphere, a small amount of helium enters the free volume of single vacancies, and thus vacancy-helium compounds form. For LaNi4.75Al0.25, helium enters the free volume of single vacancies and vacancy-helium compounds form.

Defect depth profiling after sphere indentation and blasting in aluminum and aluminum alloy detected by positron annihilation

The paper presents the experimental results of positron annihilation measurements of the subsurface zones in aluminum and aluminum alloy exposed to the indentation of the small steel ball or blasting by the silicon carbide particles. The measurements of the Doppler broadening of the annihilation line and positron lifetime (PL) enabled the depth profile of open volume defects induced by indentation and blasting processes to be obtained. The coincidence of the von Mises criterion for yield with the onset of the increase of the defect concentration at certain depth was detected. It was established that the defects created in the pure aluminum and the aluminum alloy detected by positrons are different. However, the kinds of induced defects do not change with depth. The PL measurements indicate the presence of vacancy clusters in samples exposed to blasting. The vacancy clusters created in the aluminum alloy are larger than in the pure aluminum.

Vacancies in electron irradiated 6H silicon carbide studied by positron annihilation spectroscopy

ABSTRACTPositron lifetime spectroscopy was employed to study the as-electron-irradiated (10 MeV, 1×1018 cm-2) n-type 6H silicon carbide sample in the measuring temperature range of 15 K to 294 K. Isochronal annealing studies were also performed up to the temperature of 1373 K by carrying out the room temperature positron lifetime measurement. Negatively charged carbon vacancies and VCVSi divacancy were identified as the major vacancy type defects induced by the electron irradiation process. The concentration of the VCVSi divacancy was found to decrease dramatically after the 1973 K annealing.

Permeability, permselectivity, and penetrant‐induced plasticization in fluorinated polyimides studied by positron lifetime measurements

AbstractThe ortho‐positronium (o‐Ps) lifetime τ3 and its intensity I3 in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin–lattice relaxation time T1 and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. τ3, I3, and the distribution of τ3 changed when the bulky moieties in the polyimides were changed. The polyimides, having both large τ3 and I3 values, exhibited a short T1 and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I3 and the average free‐volume hole size estimated from τ3. In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free‐volume hole properties probed by o‐Ps. The large and broad distribution of the free‐volume holes and increased local chain mobility for the 2,2‐bis(3,4‐decarboxyphenyl) hexafluoropropane dianhydride‐based polyimides are thought to be important physical properties for promoting penetrant‐induced plasticization. These results suggest that o‐Ps is a powerful probe of not only the free‐volume holes but also the corresponding permeation mechanism and penetrant‐induced plasticization phenomenon. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 308–318, 2003

Point defect behavior in electron irradiated V–4Cr–4Ti alloy

To investigate the migration behavior of interstitials and vacancies in vanadium alloy V–4Cr–4Ti, high purity V alloys were irradiated with 2 MeV electrons using a high voltage electron microscope (HVEM) and 28 MeV electrons using a electron linear accelerator. Interstitial type dislocation loops formed quickly in all cases of HVEM irradiation. The saturation density of the dislocation loops was independent of irradiation temperature and defect production rate at temperatures in the range of 323–448 K. However, the logarithm of saturated dislocation loop density was inversely proportional to the irradiation temperature, and dislocation loop density was directly proportional to the square root of the defect production rate above 448 K. The dissociation energy of the interstitial-impurity complex in V–4Cr–4Ti alloy was estimated to be 1.1 eV from the HVEM experiment. It was found from positron lifetime measurements that the vacancies form vacancy clusters at 373 K.

Defects properties in plastically deformed silicon studied by positron lifetime measurements

Positron lifetime spectra have been measured in float-zone silicon plastically deformed at different temperatures. The temperature dependences of lifetime spectra were measured for all as-deformed and annealing samples. The experimental results indicate that there are three kinds of defects (vacancy clusters, dislocation-bound vacancies and undisturbed dislocations) induced by deformation. The size and concentration of vacancy-clusters depend on the strain temperature in the case of similar strains and strain rates. Dislocation-bound vacancies were quite stable. They could not be annealed out thoroughly after thermal treatment at 1300 °C. At sample temperatures below 120 K, regular dislocation segments have a significant influence on positron trapping. Their effect as positron shallow traps influences significantly the course of the measured average positron lifetime.